1. Technical Field
The present invention relates generally to rolling element-type bearing assemblies, and particularly relates to bearing assemblies of enhanced performance and durability that are coated upon at least a portion thereof with one or more superhard polycrystalline superlattice surface coating materials.
2. Discussion
Bearings support other parts in a machine which rotate, slide or oscillate in or on them in a reduced friction, or anti-friction, arrangement. Rolling element bearings are of various types including ball bearings, roller bearings and thrust bearings. Ball bearings and roller bearings have spherical and cylindrical roller elements, respectively, disposed between two concentric ring-shaped members, or races. A thrust bearing has a pair of radial surfaces with rolling elements between, facing the two surfaces. In rolling element bearings, a plurality of roller elements, such as spherical roller elements, or balls, are confined between first and second rings, or races. In the case of a bearing which provides restraint of radial loads, the races would define inner and outer races. Free motion is accomplished between the two rings through the rolling of the roller elements against a first load support surface of a first race and a first load support surface of a second race. As such, the roller elements are sized to a rolling diameter suitable for being retained in a rolling relationship between the races. The roller elements may be retained in a spaced arrangement relative to one another through the use of one or more types of support structures, such as a cage.
Alternatively, a bearing assembly may include a plurality of elongated roller elements as well, confined within suitably sized first and second races. This type of arrangement is called a roller bearing assembly. Roller bearing assemblies are made in cylindrical configuration and in a tapered configuration, wherein tapered roller bearings are set within a correspondingly tapered set of races.
One feature that describes a superior bearing assembly is its durability. The durability for a given bearing assembly is described in the art as the rolling contact fatigue (RCF) life for the assembly. Durability of a bearing assembly is often related at least in part to the freedom of motion of the components therein. Improvements in the durability of bearing assemblies are therefore described as improvements in the RCF life. The RCF life of a bearing assembly is dependent at least in part upon several properties of individual components of the assembly, as well as upon the interaction between these individual components. These properties include those associated with the material selections for the individual bearing assembly components, such as hardness and resistance to corrosion and other chemical interaction. These properties, in turn, affect the interactions among components of a bearing assembly, through such measured properties as coefficient of friction. It is thus desirable to improve one or more of these properties, such as increasing hardness, increasing corrosion resistance and decreasing coefficient of friction, in order to improve the RCF life of a bearing assembly.
The materials from which bearing assembly components are constructed have included various metals, metal alloys and ceramic materials. Some examples include alloy steels, stainless steels and silicon nitride. The selection of base material from which bearing assembly components are constructed is important because the life of bearing assemblies is determined in large part by the physical characteristics of each bearing component's base material. For example, ceramics have been shown to exhibit superior resistance to wear as compared to steel when ceramic balls have been paired with steel races in a bearing assembly. The importance of physical characteristics of a particular material selection may relate to its interaction with other materials or its performance alone. For this reason, the selection of bearing component materials necessarily takes into account material properties both on an individual basis and within a multiple-component assembly.
With regard to individual material properties, one way in which bearings are expected to fail is by spalling, typically initiated at a subsurface location at the depth of maximum shear stress. Physical defects in the base material, especially at subsurface locations, can act as stress risers, thereby creating more favorable initiation sites for spalling to occur. Such defects are typically non-metallic inclusions that occur during steel making production.
Improvements in steel making technology over the years has caused vast improvements in the internal cleanliness of the materials from which bearing components are made. Prior to the mid-1960's steels were air-melted. Subsequently introduced vacuum degassing technology reduced the non-metallic inclusions in steel. Since the early 1980's, further improvements in the reduction of non-metallic inclusions were accomplished by the introduction of ladle refining and continuous casting of steels. In particular, ladle refining allowed for the close control of steel composition and oxygen content, while continuous casting reduced inclusions from refractories.
Since the materials from which bearing components are made, such as steels, have become cleaner, the classic failure of subsurface initiated spalling has become less frequent. The initiation of failures has therefore become attributable to surface or near surface physical properties. The hardness of the base material has become an important design consideration toward both the durability and performance of a bearing assembly. Increased hardness at the surface results in lower coefficients of friction during relative movement. Increased hardness can also prevent raceway or rolling element surfaces from becoming dented from debris rolled between contact surfaces. Debris denting can cause stress risers at the surface of a component and can act as an initiation site for failure.
Improvements have been made in the hardness of base materials through the manufacture of ceramics that are typically harder than steel. However, ceramics are often more expensive to manufacture, and have lower fracture toughness and low coefficients of thermal expansion that have to be designed differently for. Therefore, there has been a recent focus on the application of surface coatings to steel bearing components as a method for extending bearing life. Surface coatings have been found to enhance performance and durability of bearing components somewhat by supplementing the base material with a material having superior physical characteristics to those of the base material. The use of surface coatings is therefore, at a minimum, intended to produce a surface that is superior to that of the original base material at the surface level.
The application of coatings to bearing assembly components enhances several physical properties of the base material. These properties include hardness and resistance to corrosion or other undesirable chemical interaction. The hardness of the load supporting surfaces of a bearing assembly, in turn, enhances certain physical characteristics of the surface which become apparent during rolling contact. These characteristics include improved resistance to surface cracking, improved resistance to debris denting and possibly lower coefficient of friction. The resistance to corrosion or chemical interaction is typically the result of the surface coating being non-reactive and acting as a physical barrier between a potentially corrosive environment and the base bearing material.
The mechanisms by which coatings extend the RCF life of bearings are not completely understood. Recent studies have, however, recognized some reasons for the advantages realized by their use. Some coatings impart a compressive residual stress on the base material. Imparting a compressive stress thereby potentially enhances the life of the bearing. When a bearing is rolling contact fatigued, the surface of the bearing is subjected to cyclical compressive and tensile stresses. If a large residual compressive stress is present on the surface of the bearing, the tensile stress must be sufficiently large to overcome the surface compressive stress before the bearing experiences the detrimental effects of the tensile stress. Compressive stresses from coatings can thus postpone the onset of surface cracks.
The adhesion of a coating onto the surface of a bearing is also very important toward its effect on the above properties and characteristics. Many coatings that have been developed do not remain adhered to a surface when subjected to rolling contact stresses. The manner in which such coatings become removed typically involves a dusting or flaking of coating particles from the bearing surface. These particles can become lodged in or can repeatedly pass through multiple contact areas, causing resistance in operation. This resistance is exhibited as excessive noise and vibration. Resistance is detrimental to performance of a bearing assembly because it restricts freedom of motion. Resistance is detrimental to durability of a bearing assembly because it can cause premature failure through damage such as surface cracking or debris denting. In some situations, such as bearings used in the operation of silicon wafer processing equipment, it is also desirable to minimize dust or flakes, which could violate cleanliness requirements of the operations. Therefore, it is important that a coating exhibit adequate adhesion. However, it should be realized that not all well-adhered coatings enhance the RCF life of a bearing assembly. Poorly adhered coatings flake off from the base material soon during testing. The coatings of the present invention, however, are intended to have a much longer wear life.
Bearings often operate in applications having minimal lubrication. Such applications include locations that are starved of oil or other lubricant during start up, and conditions where a minimal lubricant film is available throughout operation. In both cases, asperity contact between the rolling surfaces occurs, which can lead to surface initiated damage and shorter bearing life. Therefore, it is desirable for this additional reason to have a hard load supporting surface for these bearings.
Other advantages realized by bearings having hard surface coatings include the ability to accomplish the same load support using bearings of smaller dimensions. These advantages can result in cost savings during manufacture and size and weight savings, which can affect convenience of operation and the reduction of required physical space.
Prior attempts to construct bearing assemblies with hard surface coatings have included bearings having coatings of synthetic diamond or diamond-like carbon. These coatings are deposited as atoms of carbon derived from molecules of a carbon-containing gas, such as methane. Other bearing assemblies have included various transition metal nitride coatings such as TiN, ZrN, HfN, CrN, Mo.sub.2 N, Ti.sub.0.5 Al.sub.0.5 N, Ti.sub.0.5 Zr.sub.0.5 N and (Ti--Al--V)N (from the aircraft alloy Ti-6 wt. % Al-4 wt. % V). See, for example, Thom et al., Surface and Coatings Technology, 62, 423-427 (1993) and Sproul et al., Surface and Coatings Technology, 61, 139-143 (1993). Many of these attempts at hard surface coatings have been directed toward reaching the hardness figures for diamond thin films, which range from about 55 GPa to about 110 GPa. The hardnesses of these prior surface coatings have not been satisfactory, however. For example, the hardnesses of diamond-like coatings that have exhibited favorable RCF properties have been about 11-12 GPa.
Research into hard surface coatings has also inquired into how these coatings change the coefficient of friction. The coefficient of friction is most influential toward RCF life when a bearing assembly is operated under non-lubricated conditions. Diamond-like hydrocarbon coatings have been found to substantially lower the coefficient of friction of a bearing steel to about 0.1. For applications where bearings are run in the absence of lubrication, lowering the coefficient of friction to this level or below is desirable.
Therefore, while prior bearings have included hard surface coatings, there is nevertheless a need for improvement in bearings having hard surface coatings. Improvement is needed in terms of hardness, rolling contact fatigue life, coefficient of friction, resistance to chemical interaction and freedom of motion between components, as well as in their ease and cost of manufacture. The need for durability improvements is evidenced by the fatigue and wear still experienced by bearing assembly components from their operation. The need for improvements in coating selection for bearings is evidenced by several deficiencies in the application of prior coatings to bearing assemblies. For example, some coatings cannot be synthesized at or near room temperature. The application of such coatings to bearing assemblies at elevated temperatures can result in alteration of the bearing assembly substrate properties. Additionally, some coatings require expensive or specialized equipment, and/or are difficult to be scaled up for industrial applications.
There are also considerations for bearing assembly coating materials surrounding their construction and thickness. Typically, many industrial equipment surfaces that are coated for enhancing wear and/or performance will include a relatively thicker coating than is desirable in bearing assemblies. Bearings often require thinner surface coatings because thinner coatings stay better adhered under rolling contact conditions. For this and other reasons, the principles of surface coatings for other types of surfaces cannot necessarily be extended to apply directly to bearings. The selection of bearing coating materials therefore requires an analysis of the efficiency of benefits achieved relative to thickness.
The need therefore exists for an improved bearing assembly, in terms of durability and performance, having surfaces of improved hardness, rolling contact fatigue life, coefficient of friction and freedom of motion between components, as well as in their ease and cost of manufacture. The need further exists for bearing assemblies having thin surface coatings that can impart the above advantages. The need further exists for bearing assemblies having the above advantages that are easy and economical to manufacture.